Maitotoxin, a highly potent marine toxin isolated from dinoflagellate, Gambierdiscus toxicus, has been shown to activate voltage dependent calcium channels. It has also been shown to be cardiotoxic. The mechanism is not known. In the present study we have investigated the mechanism of maitotoxin induced cell death in neonatal rat cardiomyocytes maintained in culture. Maitotoxin induced a time and dose dependent cell killing of cardiac myocytes. Cell death was monitored by leakage of the cytoplasmic enzyme lacetate dehydrogenase. Maitotoxin induced cell death was dependent on the presence of millimolar concentrations of calcium in the medium. Reduction of extracellular calcium to 100 muM markedly reduced the cell killing effect of maitotoxin. Maitotoxin markedly increased the influx of calcium in a dose dependent fashion. It caused a rapid flux of calcium into the cells which after reaching a peak declined. This reduction was due to cell death as shown by leakage of LDH at this time. The uptake of calcium induced by maitotoxin was blocked by the calcium channel blocker, verapamil. Mn2+, however, was much less effective at equimolar concentration (100 muM). Both of these blockers also inhibited maitotoxin induced cell death as measured by LDH leakage. The mechanism of cell death induced by maitotoxin appears to be related to its capacity to lower cell ATP. Lowering of cell ATP was routinely monitored by leakage of (14C)-adenine nucleotides from cells which has been prelabeled with (14C)-adenine. The leakage of adenine nucleotides occurs before leakage of LDH, but only after increased influx of calcium. The primary effect of maitotoxin on cardiotoxicity thus appears to be an effect on calcium flux. The increased calcium within the cell inhibits and/or uncouples oxidative phosphorylation resulting in a marked reduction in cell ATP which determines cell death.